Creating a Custom Brush: Step One
The first and possibly most important step is to make one single hair with the Brush Tool. This Hair should be as perfect as possible and match the other Hairs on the Face, because it will become the custom Brush. If you want to create stubble, simply make the brush a little smaller to resemble a single spot of stubble.
Create a white Background around the hair by using the Marquee Tool. Then, after selecting the boxed hair, go to Edit -> define brush preset. After it is saved, you can open it in the Brushes panel.
Creating a Custom Brush: Step Two
Spend time playing around in the brush menu! Many little Adjustments add up to an amazingly realistic look. Make changes such as increasing the Angle Jitter, Roundness Jitter, Scattering, Opacity, Spacing, etc.
A Hairier Face
When you are satisfied with your custom brush, it’s time to paint! All you have to do is paint on the Face where you want the hair to be.
Remember: hair is not always the same Color, so a good mixture of Lights and darks is important when making realistic facial hair. Hold ALT/OPT and sample multiple Colors as you go. Another secret is to paint with white to create some Highlights.